The present invention relates to the field of magnetic recording media and, more specifically, to apparatus and methods for disorienting acicular magnetic particles in the magnetic layer of a recording medium that is designed for use in fabricating magnetic recording disks, especially flexible disks.
Magnetic recording media having a magnetic layer, comprising anisotropic acicular magnetic particles dispersed in a binder, coated on a nonmagnetic support are well known in the art. In audio and video recording tapes configured for longitudinal recording, it is desirable to align or orient the magnetic particles such that they lie substantially parallel to the support in the direction of tape travel past the read/write head to achieve maximum recording density and signal output.
If, however, such an oriented tape medium is used in the fabrication of a flexible or floppy recording disk(e.g. by punching a disk out of a web of the tape medium) the disk will exhibit unacceptable variations in signal output level because of the constantly changing orientation of the aligned particles on the rotating disk with respect to the disk drive recording head which only moves radially with respect to the disk. That is, when a constant amplitude input signal is recorded on a circular recording track and then read, it will have a sinusoidal wave form which exhibits two maximums, 180.degree. apart, where the oriented particles are aligned with the head, and two minimums, at 90.degree. with respect to the maximums, where the particles are transverse to the head.
To overcome this problem, two different types of recording media are used in fabricating magnetic recording disks. In one type, the particles are circularly oriented about the axis of disk rotation so that they lie along the recording tracks. In the other type, the particles are intentionally disoriented to provide a random directional distribution of particles so that the disk drive head does not "see" any predominant particle alignment direction during the course of a complete revolution of the disk. The need to disorient the particles arises from the fact that the particles generally become somewhat oriented as a result of a coating operation. This is especially true in web coating wherein the particles tend to become mechanically aligned in the direction of web movement past the coater.
Magnetic recording media employing acicular magnetic particles dispersed in a binder generally are formed by coating, or otherwise applying, a fluid magnetic pigment or paint layer on a nonmagnetic substrate, subjecting the magnetic layer to a magnetic field while the paint is still fluid to orient or disorient the particles, as the case may be, and drying the paint to at least partially solidify the binder and fix the position of the particles therein. Also, before the binder is completely solidified the medium may be calendered, or otherwise treated, to enhance the surface smoothness of the magnetic layer.
The magnetic paint generally is a dispersion of anisotropic acicular magnetic particles, binder, and other optional additives (e.g. lubricant, abrasive, anti-static agent, dispersent, etc.) in a sufficient amount of solvent to provide the appropriate fluidized state to faciltate the coating operation. The orienting or disorienting magnetic field is applied while the paint is still fluid so that the magnetic particles may physically move and rotate within the paint in response to the applied magnetic field to achieve the desired orientation or disorientation.
In recent years, magnetic disk media have been significantly improved in terms of increased data recording density. Improvements in composition and processing of the magnetic layer has yielded increases in linear bit density (bits per inch) along the recorded track, while more precise disk drive mechanisms and servo techniques for locating the head in relation to the recording tracks have provided higher track densities (tracks per inch).
Further improvements in linear bit density are expected to be achieved through the use of smaller magnetic particles. However, to prevent self-demagnetization by adjacent opposing magnetic regions in the disk, the particles will have to be of higher coercivity and the magnetic layer will be thinner. Also, it is very important that the surface finish be as smooth as possible to minimize the head to media distance, head bounce and dropouts.
There are many contributing factors to achieving a smooth surface finish. These inlcude a surface finish of the non-magnetic support on which the magnetic layer is applied, the degree of uniformity of the paint dispersion, and the various steps in the manufacturing process.
Web coating apparatus tend to have a somewhat negative impact on smoothness in that the fluid paint layer generally exhibits a microscopic texture attributable to the particular type of coater used to apply the magnetic paint. For example, doctor blade and slot extrusion coaters tend to induce minute longitudinal striations along coating direction while gravure roll coating tends to produce a dot pattern texture.
The application of a particle orienting or disorienting field may have a detrimental affect on surface smoothness if the field agitates the particles so vigorously within paint as to cause surface blemishes. As particle coercivity increases, stronger magnetic fields will be required to move the particles.
Post drying calendering on the other hand is a positive contributor to smoothness whereby the magnetic layer is pressed against a smooth roll surface before the binder is fully cured to flatten surface irregularities.
While a recording disk medium that employs circular particle orientation may provide somewhat higher signal output than a disoriented particle medium, it tends to be very much more expensive because it does not readily lend itself to highspeed volume production.
For example, one method of fabricating a circular particle orientation disk is to mount a disk substrate on a rotating support, spray or otherwise apply a magnetic layer, and then orient the particles by subjecting the fluid layer to a magnetic field prior to solidification of the binder. Because such disks must be made one at a time, the process is inherently expensive.
Another method disclosed in British Pat. No. GB1416495, employs the steps of coating the magnetic layer on a continuous web support, applying a circular magnetic field to sequential disk area zones on the web to achieve circular particle orientation in each zone, and after drying and curing, punching a disk out of each zone. This method also tends to be expensive because the web must be stopped or moved relatively slowly at the particle orientation station or else smearing will occur thus severely limiting the quantity of coated web output for a given time period. Also, additional expense is incurred because of the tight process tolerances needed to insure that the rotational axis of the stamped disk coincides with the axis of the circular array of particles.
Disoriented particle media on the other hand has the potential for being very much less expensive in that the magnetic layer may be coated on the support at relatively high and economical web speeds. Also, because the disk punching dyes do not have to be registered with circular particle orientation zones on the web, the stamping operation may be carried out at the higher medium transport speeds to reduce costs. Additional economy may be realized in that the disoriented particle media may be fabricated on essentially the same production equipment as oriented particle video or audio tape simply by changing the particle orientation device.
Apparatus and methods for magnetically disorienting acicular magnetic particles in a fluid magnetic layer prior to drying and/or curing the layer are known in the art.
For example, U.S. Pat. No. 4,338,643 discloses a web coated magnetic recording media wherein the magnetic particles in a fluid magnetic layer on the moving web are first oriented with a magnetic field to form a herringbone pattern of adjacent 5 mm striped zones with particles in adjacent zones having a different direction orientation with respect to the direction of web movement. After leaving the orientation field, the magnetic particles in adjacent zones interact with each other at the boundries of the zones and, as time goes by, the particles become nearly random before the magnetic layer is solidified.
The herringbone pattern is established by passing the freshly coated web over a fixed array of permanent magnets which cooperate to define a static magnetic field having various field components set at different predetermined angles to the direction of web movement. Because a certain unspecified time period is required for the highly oriented particles to interact and redistribute in a random manner, it would seem this process may not lend itself to economical high volume production in that subsequent manufacturing steps, such as drying or irradiating the paint to promote at least partial curing of the binder, may not be able to be done immediately following the orientation step.
U.S. Pat. Nos. 4,208,447 and 4,271,782 are directed, respectively, to a method and apparatus for magnetically disorienting magnetic particles in a web coated medium by passing the web, with the still fluid magnetic paint thereon, over a fixed array of permanent magnets which cooperate to define a static magnetic field which diminishes in strength in the direction of web movement and includes field components of alternating direction and reversing orientation. For optimum results, the planar array of permanent magnets must be precisely spaced and inclined with respect to the plane of the fluid paint layer, a condition which may be difficult to maintain economically in a high volume production environment.
The above noted particle disorientation methods have a common feature in that both employ a complex static magnetic field through which the still fluid magnetic layer is advanced to reorder the particle distribution.
As noted earlier, it is very important in terms of performance and reliability that the surface of the magnetic layer be as smooth as possible. It has been observed that passage of fluid magnetic layer through a static disorienting field does not significantly smooth out the surface texture induced by the coating apparatus, and in some cases the field may produce additional texturing that further degrades surface quality.
As will become apparent later, the present invention provides an apparatus and method for magnetically disorienting the magnetic particles in a web coated recording medium wherein the disorienting magnetic field is not static, but rather is made to rotate in the plane of the fluid coating about an axis that is substantially normal or perpendicular to the plane. In addition to providing effective particle disorientation, this method also unexpectedly improves the surface quality of the magnetic layer by smoothing out the texture induced by the coating apparatus.
Therefore, it is an object of the present invention to provide an apparatus and method for facilitating the production of high quality magnetic recording media.
Another object is to provide an apparatus and method for improving the smoothness of a magnetic recording layer carried on a non-magnetic support.
Yet another object is to provide a method for magnetically disorienting magnetic particles in a magnetic layer of a recording medium that is to be used in the fabrication of magnetic recording disks.
Still another object is to provide an apparatus for facilitating the implementation of such a particle disorientation method.
Another object is to provide such a particle disorientation method that is compatible with economical high volume production of magnetic recording media.
Yet another object is to provide a method and apparatus that are useful in the production of a magnetic recording medium having improved physical and magnetic recording characteristics including, for example, smoothness of the magnetic layer and signal to noise ratio.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.